There is a wide variety of portable electronic devices which render audio signals in the marketplace today. Among the more popular device there are cellular phones, mp3 players, hand held video game consoles, and so on. These devices are often used by people while operating motor vehicles. To provide a safe user experience while using these devices in vehicles, manufactures have sought to develop hands-free operating modes so that the user does not have to hold the device while operating the vehicle. Some vehicles are provided with a personal area wireless network interface to allow similarly-enabled devices to transmit signals over a personal area network link to the vehicle audio system, which plays the audio signals. However, that requires both the device and the vehicle to have the appropriate components to support personal area networks.
Presently, most vehicles are not equipped with personal area network capability. However, the vast majority of vehicles do have audio systems including commercial broadcast FM radio receivers. This has given rise to a market for FM transmitter devices which connect to a portable device, and transmit an audio signal from the transmitter device over a radio channel to the vehicle's radio receiver. For various reasons, the FM band is most commonly used. Conventionally, the FM transmitter device receives the audio signal via a wired connection, such as a headphone jack. However, unlike with vehicles, personal area and local area network capability is increasingly prevalent in electronic devices, which allows for the opportunity of a wireless link between the transmitter device and the communication or computing source device, provided the transmitter device is equipped with a wireless interface for receiving the signal from the source device.
The designs of such FM transmitter devices vary substantially. Inexpensive models may have user-selectable channels settings which allow a user to select one of a small number of available channel settings, such as by a switch. Typically one or more of the selectable channels will be sufficiently clear of commercial broadcast signals to facilitate acceptable quality between the FM transmitter device and the vehicle's FM receiver. In congested urban areas, however, there may be substantial interference on all of the few selectable channels on these low complexity models. Some models allow a user to select any channel in the FM band, necessitating the user know where to find an unused channel. These type of transmitter devices also force the user to make a decision as to whether a given channel is sufficiently free of commercial FM broadcast signals by listening to the channel. The human ear is a poor judge of RF signal strength, due to the threshold effects with FM modulation. This makes it difficult for a user to manually select a weak channel suitable for the transmitter.
More complex FM transmitter devices may comprise a simple FM receiver to scan the available commercial radio band to find unused channels, and then tune the transmitter to an unused channel. It is known to simply scan the entire band and select the channel with the lowest received signal strength, or scan until a channel with a received signal strength below a defined threshold is found and use that channel. While these type of FM transmitter devices will usually find clearer channels than may be available with simpler FM transmitters that do not scan, the selected channel quality may degrade faster than other suitable channels depending on the movement of the vehicle, necessitating the user to prompt a rescan when the audio quality degrades to an unacceptable level due to interference, such as from commercial broadcasts on the selected channel. Often the new channel selection is far away from the present channel, which may require many button presses by the user to tune in the new channel. Therefore there is a need for a more effective way of selecting a radio channel for transmitting to a nearby radio receiver.